Cardiac arrhythmias are a leading cause of morbidity in the Western hemisphere. The risk of developing malignant ventricular tachyarrhythmias is associated with the extent of myocardial injury and is believed to be the primary cause of approximately 50% of all cardiovascular deaths (Myerburg R J, Kessler K M, Castellanos A., Circulation January, (85) I suppl:I2-10, 1992.). Bradycardia and heart block, which can result from the normal aging process, further adds to the morbidity associated with cardiac arrhythmias and results in the permanent implantation of over 160,000 pacemakers annually in the United States.
Conventional medical therapy is predominantly palliative treatment and commonly fails to impede and prevent the morbidity and mortality associated with cardiac arrhythmias. Radiofrequency catheter ablation of ischemic ventricular tachycardias is considered adjuvant therapy rather than curative. The implantation of defibrillators and pacemakers, while generally effective, does have problems which include: (1) implantation of a mechanical device and its need for replacement every 4 to 7 years, (2) surgical and mechanical complications resulting from the implantation of the device, (3) negative physical and psychological effects of an implanted mechanical device and (4) a prevalent need to use concurrent antiarrhythmic therapy and/or radiofrequency modulation/ablation.
In some instances, and especially where the conduction disturbances are due to ischemia, only more radical options are available, such as surgery. However, even surgical techniques can fall well short of the therapeutic goal of restoring cardiac function in the patient. For example, coronary bypass surgery is frequently inadequate to restore function in patients who have few viable surviving myocytes in the infarct region. Therefore, there is a need to develop alternative therapies for treatment of myocardial dysfunction that overcome the negative aspects of current treatment methods. In contrast to the conventional treatment modalities which attempt to simulate the physiological process of the heart, the application of tissue engineering to correct conduction disturbances would enhance the natural physiological processes.
Tissue engineering techniques are attractive alternatives to such conventional therapies. Tissue engineering techniques generally involve transplanting cells that can imitate certain cardiac functions into cardiac tissue to effect myocardial repair (Soonpaa, M., Koh G Y, Klug M G, Field L J, Science, 1994. 264: p. 98-101; Orlic D, Kajstura J, Chimenti, S, Jakonluk I, Anderson S M, Li, B, Pickel J, McKay, R, Nadal-Ginard, B, Bodine, D, Leri A, Anversa P, Nature (2001) 410:701-705. Chiu R C-J, Zibaitis A, Kao R L, Ann Thorac Surg (1995) 60:12-18).
Tissue engineering techniques involving, for example, transplantation of skeletal myoblasts to effect myocardial repair have gained increased attention with the demonstration that skeletal myoblasts survive and form contractile myofibers in normal and injured myocardium (Weisel R D et. al., J. Thoracic Cardiovascular Surgery 2001, 121:835-836; Murry, C., Wiseman R W, Schwartz S M, Hauschka S D, J Clin Invest, 1996. 98: p. 1512-2523; Murry C E, Wiseman R W, Schwartz S M, Hauschka S., J Clin Invest (1996) 98:2512-2523). Cell transplantation and tissue engineering of skeletal myoblast, and stem cells offer the promise of restoring function to patients with limited available myocytes. However, the emphasis of myocardial repair to date has focused on the preservation of myocardial contractility with little attention given to the effects of tissue engineering on cardiac conduction. One concern with the use of skeletal myoblasts transplantation for myocardial repair is whether the skeletal myoblasts will propagate electrical activity to cardiomyocytes.
Cardiomyocytes are electromechanically coupled by intercalated disks composed of adherens and gap junctions. N-cadherin is the major adherens junction protein, whereas connexin 43 (Cx43) is the major gap junction protein in the ventricular myocardium (Verheule S et. al., Circ. Res. 1997, 80:673-81). Due to the difference of cellular electrophysiological properties of cardiac cells and skeletal muscle cells, tight coupling of cardiac and skeletal muscle cells are required for synchronized electrical communication (Lee et al., Annals of Biomedical Engineering 28-1:S54, 2000).
Skeletal myoblasts express N-cadherin and connexin 43 as replicating myoblasts and then downregulate the expression of these two proteins after differentiation and myotube formation. Functional gap junctions have been detected during the early stages of skeletal muscle development, and gap junction intracellular communication has been suggested to play an important role in myoblast fusion and differentiation (MacCalman, C. D. et. al., Dev. Dyn. 1992, 195:127-132). Although multiple studies have shown that skeletal myoblasts survive cardiac grafting and form myotubes, these studies have not shown whether skeletal fibers form functional junctions with the surrounding cardiomyocytes allowing for electrical communication between the host and grafted cells. Most of these studies have indicated that connexin 43 (Cx43) and N-cadherin are not detectable in the skeletal muscle cells grafted into the host myocardium after cellular differentiation (myotube formation) by the lack of electromechanical coupling between grafted cells and myocardial cells (Murry C E et. al., J. Clin, Invest. 1996, 98:2512-2217; Robinson et. al., Cell Transplantation 1996, 5(1) 77-91; MacCalman, C. D. et. al., Dev. Dyn. 1992 195:127-132; Knudsen, K A et. al., Exp. Cell Res. 1990, 188:175-184; Balogh, S. et. al., Dev. Biol. 1993, 155:351-360; Dahl, E. et. al., Anat. Embryol. 1995, 191:267-278). Previous attempts to transplant skeletal muscle cells into myocardium have lacked the electrical coupling to cardiac cells which is necessary for myocardial coordinated activity.
When skeletal myoblasts and cardiomyocytes, or myotubes and cardiomyocytes, are co-cultured in vitro, the cells were found to be electromechanically coupled (Reinecke, H. et. al, J. Cell Biology, 2000, 149(3), 731-740). Reinecke et al. reported that cardiomyocytes were capable of forming electromechanical junctions with some skeletal myotubes in vitro and induced their synchronous contraction via gap junctions. N-cadherin and connexin 43 were both detected at the contact sites between cardiomyocytes and skeletal myotubes in this in vitro study, although the roles or importance of these proteins, or the mechanism involved, in forming gap junctions remained un-determined. While these studies exemplify the association of connexin 43 expression and functional gap junctions with cardiomyocytes in vitro, no evidence is presented which indicates that adult skeletal myocytes, which have minimal Cx43 expression, would be capable of forming functional gap junctions in cardiac tissue.
Accordingly, there is a need in the field to provide methods and compositions for induction and enhancement of the electrical coupling between cardiomyocytes and transplanted cells, such as adult skeletal muscle cells, to effect cardiac repair.